Using long-term occupancy information to inform the management of Cape Sable seaside sparrows in the Everglades

Abstract

Wildlife managers must find ways of detecting meaningful shifts in population size or extent if they are to effectively implement recovery programs for threatened and endangered species. This task is made difficult by the oft-flawed survey information to which they have access. Here we describe such a situation for the Cape Sable seaside sparrow, an Everglades endemic. An unusually long-term survey has been conducted across this subspecies’ entire range, however extracting abundance information from this survey has proven difficult and controversial. Here we treat the information contained in the survey as presence–absence data and evaluate occupancy trends through time. We also evaluate the probability that each survey site has been effectively abandoned by sparrows and the manner in which site utilization varies through time and across space. Our results indicate that Cape Sable seaside sparrows have experienced two episodes of occupancy declines. Two subpopulations continue to decline in occupancy to the present, and only one shows a consistent upswing. In accordance with these trends, we also show that the two populations that have a decreasing trend in occupancy also hold the vast majority of sites that have been abandoned by sparrows. Finally, we show a dynamic pattern of site utilization when sparrow occupancy is low and a coalescence of site usage as occupancy increases. Overall our results confirm previous proclamations of severe decline in the Cape Sable seaside sparrow numbers and argue for continued vigilance in ensuring no further reductions. We also illustrate how presence–absence information can be used to extract quite detailed information on changes in habitat occupancy and site usage that are highly relevant to managers.